His voice dropping to denote displeasure with the thought, Israeli Defense Minister Ariel Sharon looked earnestly at his audience of American Jews. ''Imagine the situation,'' he said, ''if we withdraw - after everything that has happened - and our northern towns and cities are shelled. What are we going to do? Go again into Lebanon?''
Moments later, matter-of-factly but no less bluntly, Israeli Foreign Minister Yitzhak Shamir told delegates to the Israel Bonds convention: ''We are concerned what will happen the day after our withdrawal. The PLO will try to return. The Lebanese Army is too weak to stop them.''
Enduring security - that is the overriding theme expressed by Israeli government officials as the Israeli-Lebanese-American negotiations on the future of Lebanon continue, deadlocked, into their second month.
Besides the universally agreed upon need for secure borders for Israel, the government's own credibility is at stake. The speeches Jan. 26 by the second and third most important men in the Israeli Cabinet, coming at a time of strained relations with the United States, provide a valuable insight into official Israeli thinking. And they seem to indicate little give in the Israeli position.
The costly, image-tarnishing war in Lebanon was conducted, the Israeli leaders have contended, to drive the Palestine Liberation Organization out of Lebanon once and for all and to achieve the much-promised ''peace for Galilee.'' If the north of Israel is harassed in the future (there have been three incidents of Katayusha rockets being fired, harmlessly, into Galilee from southern Lebanon in the past three weeks), the Menachem Begin government is in trouble. Mr. Sharon, in particular, has been associated with the war in Lebanon as the architect of the invasion.
Mr. Sharon told his audience Israel is holding fast to four basic conditions necessary before the Israelis agree to a settlement. He indicated there is little room for compromise on these conditions since they involve matters of ''life and death'' for Israel: